Reciprocating hermetic compressors comprise a cylinder block, which is mounted inside a hermetically closed shell and which is secured to said shell through an adequate dampening means, said cylinder block having a main bearing, which receives and supports a flange portion of a rotor shaft which is concentrically placed in a central bore provided in the main bearing of the cylinder block. A lower end portion of the rotor shaft, projecting from the main bearing, is rigidly mounted to the rotor of the electric motor of the compressor. In reciprocating compressors, said shaft moves parts of a connecting rod--piston subassembly during its rotation caused by the rotation of the rotor, said shaft providing at the same time the pumping of the lubricant oil to the movable parts of the compressor.
The mounting of the shaft to the rotor is achieved in order to avoid the formation of gaps between said parts, avoiding relative movements between the shaft and the rotor.
One of the known processes for mounting the electric motor shaft of the above cited type uses mechanical interference at room temperature or with rotor heating. In the mounting in which there is little or no rotor heating, the shaft is introduced with a great effort into the central bore of the rotor, causing mechanical wear of said shaft and rotor parts, which usually deforms the resilient components or fragile component parts of the assembly.
In the mounting under high temperature, the shaft is introduced into the rotor when said rotor is heated to a temperature enough to cause its dilatation and consequent diameter increase of the central bore. The fixation of the shaft to the rotor, after said shaft has been introduced in said rotor, occurs during cooling and consequent contraction of said rotor, which, by reducing the diameter of the bore, eliminates the mounting gap and adjusts the rotor to the shaft. This type of assembly damages the components which are delicate or susceptible to deterioration caused by heat in the rotor.
In order that the assembly with rotor heating does not require great efforts, it is necessary to increase the temperature of the rotor. Nevertheless, in ECM motors, in which the rotor carries magnets, such temperatures cause damages to the rotor, since it is still required a determined degree of mechanical interference, or because the high temperature, which is necessary to eliminate the mechanical interference, destroys the rotor magnets.
This effect is particularly important when the magnets are affixed around the rotor core.
These problems are intensified when part of the components involved are obtained, e.g. by sintering, such as the rotors formed with sinterized magnets, which besides being fragile do not resist to high temperatures or high thermal gradients, the characteristics of said rotors being impaired when the temperature is higher than 120.degree. C.
Moreover, high temperatures are also infeasible in solutions in which a rotor cover of the stainless steel type or a coiled cover is used. Although the stainless steel covers support high temperatures, in this solution the heating destroys the resin which retains the magnets on the rotor core. The coiled cover cannot withstand temperatures above 150.degree. C.
Another inconvenience of the known solutions comes from the need for high and costly dimensional precision of the shaft-rotor parts, in order to guarantee an acceptable degree of mechanical interference during the assembly, maintaining the heating temperature of the rotor within acceptable limits.